The present invention relates to a fluid displacement mechanism of the gerotor type, and more particularly, to an improved coupling for use therewith.
Gerotor fluid displacement mechanisms (gear sets) have become quite popular, and their commercial use very widespread. Gerotor gear sets are used typically as the fluid displacement mechanism in low-speed, high-torque hydraulic motors, and the present invention will be described primarily in connection therewith. However, those skilled in the art will understand that the use of the invention is not so limited, and it may be applied advantageously to other devices utilizing a gerotor as the fluid displacement mechanism. For example, a gerotor is used as the fluid meter in a full fluid linked hydrostatic power steering unit, an example of which is illustrated and described in U.S. Pat. No. Re. 25,291, assigned to the assignee of the present invention, and incorporated herein by reference.
Most low-speed, high-torque gerotor motors made commercially are of either the "spool valve" type, illustrated and described in U.S. Pat. No. 4,171,938, or the "disc valve" type, illustrated and described in U.S. Pat. No. 4,343,600, both of which are assigned to the assignee of the present invention, and incorporated herein by reference. In either case, the star member of the gerotor gear set orbits and rotates within a stationary ring member, such orbital and rotational movement providing the low-speed, high-torque output, as is well known to those skilled in the art.
Unfortunately, the orbital and rotational movement of the gerotor star, in and of itself, is generally not useful, but must first be translated into pure rotational movement of a member, such as a motor output shaft. In the case of a hydrostatic power steering unit, the orbital and rotational movement of the gerotor star must be translated into rotational movement of a follow-up valve member, as is well known in the art. For as long as low-speed, high-torque gerotor motors have been known, the typical, commercial product has utilized a "dogbone" shaft to transmit the orbital and rotational movement of the star into rotation of the output shaft. Such dogbone shafts are illustrated and described in the above-incorporated patents. The conventional dogbone shaft is a solid shaft, and has a set of external, crowned splines at each end, one set being in splined engagement with straight internal splines defined by the gerotor star, and the other set of crowned splines being in splined engagement with straight internal splines defined by the output shaft. The crown of the external splines on the dogbone shaft permits it to "wobble", with the end engaging the star orbiting and rotating, while the end engaging the output shaft merely rotates.
Although the dogbone shaft and spline arrangement described above has been quite successful commercially, in terms of general motor performance, durability, etc., the arrangement does have a number of disadvantages, which have traditionally been considered somewhat unavoidable. The need to form (hob, roll, cold forge, etc.) four sets of splines per motor (with two being crowned, and one typically disposed in the bottom of a blind hole), has added substantially to the overall cost of the motor. As an additional item of cost, the star needs to be heat treated, only because of the splines, and such heat treating frequently results in distortion of the star. This potential for distortion has, until the time of the present invention, deterred those working in the gerotor art from using "net shape" powdered metal stars in their gerotor gear sets.
In addition, the rubbing action between the internal and external splines, as the dogbone shaft wobbles, generates a substantial amount of heat within the motor, which is typically transferred to the hydraulic fluid, thus increasing the need to cool the fluid, such as by means of a heat exchanger disposed somewhere in the hydraulic circuit. An increased heat load in the hydraulic circuit always adds to the overall cost of the circuit, or of the vehicle, or of the piece of equipment using the circuit.
A further disadvantage of the prior art dogbone and spline arrangement is that, in many motors, the need to reduce the wobble angle of the dogbone, for reasons well known to those skilled in the art, has resulted in a dogbone shaft having a length which makes the motor much larger in overall size than is really necessary, thus adding further to the weight and cost of the motor. In some vehicle applications, there is insufficient room for the gerotor motor which is needed, in terms of torque capacity, for the particular application.
The conventional internal splines in the output shaft/spool valve assembly results in the spool valve either being larger in diameter, thus increasing the possibility of leakage, or being thinner radially. In the latter case, under high pressure, the spool valve compresses radially, again resulting in increased leakage and loss of volumetric efficiency. In either case, the internally splined output shaft limits the potential performance of the device.
Finally, there are many potential applications for gerotor motors of the "thru-shaft" type, i.e., having an output shaft extending out of each end of the motor, with both being powered by the same gerotor gear set. It does not appear that, as of the filing of the present application, there are any commercially available thru-shaft gerotor motors. One of the possible reasons is the difficulty of transmitting orbital and rotational movement of the gerotor star into rotational movement of two oppositely disposed output shafts, without the resulting motor becoming so large and expensive as not to be economically feasible.